OK, fixed.For anyone having the same issue, here goes....
You need to put the following lines into the <system.web> section of your web.config..
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
This changes the "System.InvalidOperationException: Request format is unrecognized for URL unexpectedly ending in '/GetCategories'. " error to:
System.InvalidOperationException: Request format is invalid: application/json; charset=utf-8. at System.Web.Services.Protocols.HttpServerProtocol.ReadParameters() at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest
To fix that you then need to add the following lines of code to the <handlers> section of the web.config file.
<add name="ScriptHandlerFactory" verb="*" path="*.asmx" type="System.Web.Script.Services.ScriptHandlerFactory" />